Digital communication systems in which one or more master stations communicate with a plurality of terminals over a single communication line is an art recognized concept. For example, U.S. Pat. No. 3,821,706 discloses a system similar to that mentioned above in which a plurality of remote terminals are connected to a central computer through a full duplex communication channel. Each terminal has a register which stores the unique address for that terminal, a counter, and a comparator circuit for determining the identity between the state of the counter and the address. The computer transmits messages to all of the remote terminals consisting of digital word groups preceded by a particular terminal address. These messages are received by all of the terminals and are only processed by the terminal so addressed. All of the terminals advance their counters in response to each word transmitted by the computer, whether that word is addressed to them or not. In the absence of a message to be sent to the terminals, the computer transmits an idle word which advances all of the counters. Upon the occurrence of identity between its counter and its address, a terminal will transmit any available message to the computer on the return channel. Periodically, a word is sent out by the computer resynchronizing all of the counters.
In one alternate embodiment, the central station communicates with a plurality of satellite stations, each of which services a plurality of terminals and interrogates them in sequence, one each time that satellite's counter coincides with its address. In another embodiment, all of the terminals connected to a particular satellite are sequentially interrogated as the counter in the satellite is advanced by sequential words generated by the computer, and then the computer transmits the address of another satellite causing its terminal to be sequentially interrogated.
The present invention comprises an improvement over U.S. Pat. No. 3,821,706, the entire disclosure of which is hereby incorporated by reference. In some communication applications, it is desirable for one or more remote terminals to be able to communicate with any of a plurality of computers or master stations over a single communication line. In this type of installation, each computer or master station would communicate with one or more of the terminals over a particular data channel, with the several communication channels being multiplexed over the communication line. Also, it is desirable for any one of the terminals to be able to communicate with any of the master stations, however, this has not been possible heretofore in systems of the type described above since the transmit and receive frequencies of a particular terminal are not compatible with each of the master station's channels. Thus, in the past, it was not possible for a terminal to switch channels so as to communicate with any of the master stations.